Protein biomarkers have gained rapid increasing research interests in recent years. Detection of the expression level of protein biomarkers in patient samples enables early disease diagnosis and treatment response assessment. Protein biomarkers for cancers, heart diseases and infectious diseases have been reported to date.
Mass spectroscopy, 2D (two-dimensional) western blotting, 2D (two-dimensional) gel electrophoresis and enzyme-linked immunosorbent assay (ELISA) are the most widely used detection methods for biomarker discovery. Nevertheless, these existing techniques are not compatible with high throughput multiplex analysis. Furthermore, they are all labeling methods, where the use of fluorescent labels or secondary antibodies can interfere with the original bio-molecular interactions.
Recently, micro-array technology has become an effective alternative for high throughput profiling. The applications of antibody microarray for high throughput proteomics studies and antibody have been demonstrated, and is by far the most widely used probe for biomarker profiling. Direct target labeling, sandwich assay and competitive adsorption assay are common strategies for antibody microarray detection, in which fluorescent labeling, secondary antibodies or enzyme are required in the detection process. In antibody array detection, different fluorescently labeled molecules or secondary antibodies are required for each array element. Large scale multiplexed measurements are therefore limited by the availability of corresponding fluorescent labels or secondary antibodies.
Surface plasmon resonance (SPR) is a non-labeling detection alternative. It measures the refractive index changes associated with bio-molecular bindings occurring at the gold sensing surface. Conventional SPR imaging sensors measure the intensity distribution of the reflection beam with a CCD (charge-coupled device) detector. The major limitation of conventional intensity SPR imaging sensors is the limited sensor resolution (10−5 RIU). In recent years, spectral based SPR imaging sensor with the scanning of surface plasmon excitation minimum is reported, however the sensor resolution is limited (10−5 RIU) and time-consuming scanning is required during measurement. In the inventors' previous research, the inventors imaged the color texture variations caused by the SPR absorption minimum shift and the system was applied to image the refractive index changes of lubricant at highly pressurized contact. Nevertheless, the sensor resolution is also limited.